It has been proposed to heat the windshield of an automotive vehicle to defrost the windshield. It has been found that in cold weather the water defrosted from the windshield can run down and freeze onto the windshield wipers, thereby immobilizing the wipers. It is thus desirable, for such a system, to provide heated wipers to prevent this from happening. In addition a heated wiper will also melt ice accumulated on it during heavy snow storms to insure a functional wiper system. Even in the absence of snow and ice the wiping characteristics of a blade deteriorates at low temperatures, and a heated wiper blade would allow more consistent wiping efficiency over a temperature range.
It is known to make heated wiper blades with non-conductive rubber containing a resistive wire or other heater element for heating by an applied current. The heat is generated in the wire and must flow across the wire/rubber interface to heat the wiper. Excessive heating will occur next to the heating wire which would lead to the degradation of the rubber and a lower life for the blade. Another disadvantage is the need for a sophisticated circuit to control the heating of the blade and to prevent a runaway rise in temperature.
The use of a conductive elastomer as a wiper blade directly heated by electricity overcomes the difficulties encountered in using a wire heater; the current would flow all through the conductive blade to generate the heat in the blade itself. The conductive elastomer is inherently self limiting as a conductor: the resistivity increases with temperature and reaches very high values to limit the heating effect such that the temperature stabilizes at some value depending on the elastomer composition. This self limiting feature would prevent undesirably high temperatures and is accomplished without the use of a regulator or a complex circuit.
It is known to use carbon black in the formulation of rubber or other elastomer to make the elastomer conductive. To induce sufficient conductivity for a heated wiper, a high concentration of carbon black is required. Such a high concentration of carbon black will increase the modulus and the hardness of the rubber so that it is no longer suitable for a wiper blade application. Typical commercially available wiper blades have a hardness between 50 and 70 Shore A. A properly compounded natural rubber made conductive using only carbon black would have a Shore A durometer of 90. Such a hard material would not be acceptable for wiper blade applications.
In the formulation of rubber, plasticizers such as oils are used to soften the material. Normally the addition of such plasticizers reduce the conductivity of carbon black filled elastomers. However, we have discovered that the addition of low concentrations of plasticizers sometimes improves the conductivity. Further, we have discovered that the addition to the carbon black of graphite in low concentrations also improves the conductivity so that the amount of carbon black can be reduced, although larger concentrations of graphite decrease the conductivity. Thus by combining the discovered effects of graphite and plasticizing oils a conductive elastomer of suitable hardness can be formulated.